The electrolysis of aqueous solutions of alkali metal salts has been commercially conducted for many years. Originally, electrolytic cells were constructed using liquid mercury as one electrode and graphite slabs as the other electrode. With the introduction of new materials for cathodes and anodes, the design of the cells incorporating these materials has improved both in terms of the efficiency of operation as well as the efficiency of construction. The introduction of cell separators, such as diaphragms and membranes, into the cell structure, provided design of electrolytic cells with separate catholyte and anolyte compartments which permit the isolation and collection of the products produced by the electrolysis reaction.
The products produced in the electrolysis of alkali metal halide salts are primarily chlorine, hydrogen and the alkali metal hydroxides. The commercial markets for these products are not always in the quantity as they are produced and considerable effort has been expended to design cells that will produce the most saleable ratio of products. In addition, the quantities of electrical energy used in the commercial operations are very large and considerable efforts to reduce the electrical consumption have been undertaken.
A typical reaction in a cell using sodium chloride as the alkali metal salt for illustration purposes is: EQU 2NaCl+2H.sub.2 O.fwdarw.2NaOH+H.sub.2 +Cl.sub.2 Equation I
In Equation I, the sodium chloride solution, upon electrolysis, yields sodium hydroxide, chlorine and hydrogen. The chlorine being released at the anode while the hydrogen is released at the cathode. By maintaining a high concentration of sodium chloride in the electrolyte, the solubility of the chlorine and hydrogen is reduced so that the gas will leave the electrolyte and can be collected.
It has been shown that the power requirements for such a cell can be decreased if a sufficient supply of oxygen is maintained at the cathode to prevent the evolution of hydrogen. Therefore, through the use of membrane or diaphragm type cells, the cathode can be isolated into its own compartment and oxygen passed into that compartment to reduce the amount of hydrogen generated as shown in Equation II. EQU 4NaCl+2H.sub.2 O+O.sub.2 .fwdarw.4NaOH+2Cl.sub.2 Equation II EQU 2H.sub.2 O+O.sub.2 +4e.sup.- .fwdarw.4OH.sup.- Equation III
Equation III above demonstrates the reaction for generating hydroxyl ions. The presence of these hydroxyl ions at the cathode will cause a reaction with the sodium ions to give sodium hydroxide, and will preclude the reaction of sodium with water to produce hydrogen gas and sodium hydroxide. Equations II and III are utilized in this application to illustrate how the novel electrolytic cell design of this invention operates to produce minimal quantities of hydrogen and utilizes the depolarized oxygen cathode to produce a relatively concentrated sodium hydroxide solution.
The evolution of electrolytic cells for the electrolysis of alkali halides is described in "Electrochemical Engineering" by C. A. Mantell (4th Edition, McGraw-Hill Book Co.). Additional modifications of cell design are also described in U.S. Pat. No. 3,859,196 to Reuthel et al., U.S. Pat. No. 4,025,405 to Dotson et al., as well as in U.S. Pat. No. 4,017,376 to Mose et al., and U.S. Pat. No. 4,181,776 to Lindstrom, which illustrate the current state of the art of electrolytic cells.
Various methods have developed using porous cathodes in combination with an oxidizing gas to depolarize the electrode in electrolytic cells. Juda, U.S. Pat. No. 3,124,520, as well as Gritzner, U.S. Pat. Nos. 3,926,769, 4,035,254 and 4,035,255, Dotson et al., U.S. Pat. No. 4,035,405 and Butler et al., Canadian Pat. No. 700,933, all disclose cells wherein the cathode is depolarized. In these patents, an anolyte compartment containing the liquid anolyte, is separated from a catholyte compartment containing liquid catholyte, by a membrane or diaphragm which permits the alkaline metal ions from the anolyte compartment to pass into the catholyte compartment. In the prior art methods, the cathode is porous, and oxygen or an oxidizing gas is brought in contact with the cathode to cause depolarization at the cathode surface. The prior art cells which utilize depolarized cathodes require a distinct intermediate catholyte compartment which is not required in the present invention. In processes employing such cells, additional water is required in the catholyte compartment, and either pure water or a solution of the alkali metal hydroxide must be added in order to maintain the proper level of the catholyte in the catholyte compartment.
It is thus an object of this invention to permit the construction of an electrolytic cell of more compact design in comparison to conventional cells by eliminating the requirement for a separate catholyte compartment and external H.sub.2 O feed lines associated with the catholyte compartment.
Another object of this invention is to reduce the electrical energy needed for the electrolysis of alkali metal halide solutions in an electrolytic cell.
A further object of this invention is to reduce the liquid flooding problems associated with prior art air cathodes.
A still further object of this invention is to provide an electrolytic cell capable of producing an alkali metal hydroxide solution of relatively high concentration and purity.